I/Q (In-phase/Quadrature) modulators and demodulators are widely used in digital communications systems and are abundantly discussed in the technical literature. See, for example, Behzad Razavi, RF Microelectronics, Prentice Hall (1998) and John G. Proakis, Digital Communications, McGraw-Hill (1995). Examples of system applications that incorporate and standardize I/Q modulation and demodulation include the GSM (Global System for Mobile Communications), IS-136 (TDMA), IS-95 (CDMA), and IEEE 802.11 (wireless LAN). I/Q modulation and demodulation have also been proposed for use in short-range radio systems such as Bluetooth wireless communication systems.
Typically, in a receiver system that incorporates I/Q demodulation, the modulated carrier is simultaneously applied to an I-channel mixer and to a Q-channel mixer. A local oscillator (LO) is also applied to the mixers to effect frequency conversion to an intermediate frequency (IF). In an I/Q demodulator, the LO signal that is applied to this Q-channel mixer is offset by 90° from the LO signal that is applied to the I-channel mixer.
Image rejection is among the significant metrics by which the performance of a receiver system may be evaluated. In general, image rejection refers to the ability of the receiver to reject responses resulting from RF signals at a frequency offset from the desired RF carrier frequency by an amount equal to twice the IF of a dual-conversion receiver. For example, if the desired RF signal is at 100-MHz, and the receiver IF is 10.7 MHz, then the receiver LO will be tuned to 89.3 MHz. However, as is well known to those skilled in the art, the receiver will also exhibit a response to undesired RF signals (i.e., image signals) at a frequency 10.7 MHz below the LO frequency, in this case, 78.6 MHz. The receiver's response to the 78.6 MHz signal is referred to as the image response, because the image signal resides at a frequency opposite the LO frequency from the desired RF carrier, and offset from the LO frequency by the magnitude of the IF.
In the context of I/Q demodulator receivers, image rejection performance is known to be adversely affected by mismatch that is inevitably introduced between the I-channel and the Q-channel of the demodulator. In general, one or more of three distinct sources of mismatch may subsist between the I-channel and the Q-channel.
First, some degree of phase mismatch may be contributed by the LO signal. That is, the respective LO signals applied to the I-channel and to the Q-channel may not be offset by precisely 90°. Second, there may exist some mismatch in gain between the two channels. Gain mismatch may derive, for example, from differences in the conversion gains of the two mixers, and from asymmetry in the performance of gain stages, if any, in the respective channels. Differences in channel gain may also result from differences in the characteristics of the ADCs (analog-to-digital converters) in the channels. Third, there may exist delay (or timing) mismatches between the channels. Two sources contribute to timing delay: first, group delay differences between the respective channel filters and, second, sampling-time mismatch between the two ADCs, which, conventionally, are driven by a single ADC clock.
Although it is not uncommon for high-performance image-rejection mixers to incorporate some form of gain and/or phase mismatch compensation, there appears no entirely satisfactory technique to correct timing mismatch, particularly for systems in which the IF is relatively high, say 40 MHz or so. Accordingly, timing mismatch persists as a concern in the many emerging receiver system designs that are predicated on such IFs.
In receiver systems that incorporate a high IF, timing mismatch is particularly detrimental in that it becomes a source of significant degradation in image rejection. This is because, as the IF increases, the phase mismatch that results from a given timing mismatch increases accordingly. Conventional phase-mismatch correction techniques are unavailing as a solution. That is, available phase-correction techniques exhibit a capability to compensate for frequency-independent phase mismatch, such as, for example, a departure from the nominal 90° phase shift that is imparted to the LO inputs to a quadrature demodulator. However, timing mismatch is related to a nonideal phase/frequency response, specifically, a phase response that does not vary linearly with frequency. Accordingly, a constant phase correction that is effective at a predetermined frequency of operation is unavailing in many systems where the channel bandwidth (e.g., 45 MHz) is comparable to the IF (e.g., 40 MHz). In addition, the use of a digital complex equalizer tends to be cost prohibitive and, nevertheless, only marginally effective. Accordingly, at present there appears no readily available technique to compensate for timing mismatch in I/Q demodulator systems.